The really cruel thing is that bats aren't even blind...

July 30, 2011 6:18 AM

I have a bifocals prescription, but what I want are two separate pairs of glasses. Which bits of my prescription should I give Zenni to produce reading glasses that will actually work?

This is what my prescription looks like:

OD
Sphere: -8.25
Cylinder: -2.25
Axis: 116
Horizontal Prism: blank
Vertical Prism: blank
Add: +1.25

OS
Sphere: -6.00
Cylinder: -2.00
Axis: 057
Horizontal Prism: blank
Vertical Prism: blank
Add: +1.25

I know that the "sphere" part is the "geez, Sarah can't see much of anything at a distance" bit, so it's irrelevant for reading glasses. And I know that "add" is the basic reading glasses bit. I just don't know how important the cylinder and axis parts are. I gather they have to do with my astigmatism and so forth (and yes I am bitter about this aspect of my genetic heritage.)

My initial impulse is to tell Zenni that my prescription, for these reading glasses, should be:

OD
Sphere: +1.25
Cylinder: -2.25
Axis: 116

OS
Sphere: +1.25
Cylinder: -2.00
Axis: 057

But maybe I'm wrong - maybe the cylinder and axis aren't important and I can get +1.25 glasses from the drugstore or whatever and that'll work fine? I tried a few different combinations of search terms and feel somewhat ashamed that I can't find this answer on my own.

posted by SMPA to health & fitness (7 answers total)

See here. It's seems you are right.
posted by bluefrog at 7:15 AM on July 30, 2011

I think your reading glasses should be:

OD
Sphere: -7.00
Cylinder: -2.25
Axis: 116

OS
Sphere: -4.75
Cylinder: -2.00
Axis: 057

I wear glasses, and have ordered glasses from Zenni, but I am not your optician :)
posted by DarkForest at 8:02 AM on July 30, 2011

Yes DarkForest has the right idea. Your sphere of -8 (in your worse eye) means without glasses you can't focus past 1/8 meter (5 inches). If you put on sphere=+1.25 glasses you make that even worse (under 4 inches). You could try some at the drug store to see this effect. This is why the reading prescription is an "add": you add it to the sphere number to get the reading value.

If you're the experimental type, and considering how cheap online glasses are, you might try a third single vision set halfway in between (add +0.5 or +0.75 to your sphere values in both eyes) and consider them your "indoor" glasses. You'll sacrifice a tiny bit of sharpness at distances greater than 1/0.5 (2) meters, and you'll sacrifice a bit of your nearest focus range, but they might be a happy compromise in an indoor/office environment. You wouldn't want to wear them for driving or sightseeing in the mountains, but for watching TV or surfing, they might be fine.

(unsolicited advice:) At your high prescription value, beware polycarbonate lenses, which will give you clear vision just at the center of the lens. I like to stick to full thickness plastic (aka CR39) and use small frames to avoid lens edge thickness.
posted by fritley at 9:07 AM on July 30, 2011 [4 favorites]

IANAO; just a bifocals wearer. Consult yours.
That said, the sphere is your scrip for distance. The add is what needs to be added to the sphere that to get your reading scrip. Cylinder and axis are the corrections for astigmatism and presumably need to be included in both pairs. So, what DarkForest said.
posted by TruncatedTiller at 9:08 AM on July 30, 2011

Zenni has an "Understanding Rx" page that describes the features of an eyeglass prescription and how to convert a multi-focal prescription into various other prescriptions (i.e. single vision distance, single vision reading, single vision computer, and multi-focal computer/desktop glasses). For single vision reading glasses they say:

Combine the Add and Sph numbers for each eye. The result creates the new Sph values for each eye. In the case of a Positive Add and a negative Sph, net the difference, and the new number becomes the Sph. If your Sph is already positive, you simply combine the positive Sph and the Add number, using it for the new Sph. The Add data entry field will be blank, by virtue of it having been combined with the Sph number. Enter Cyl and Axis values for both eyes- these values are not changed in any way. For single vision reading only, narrow the distance vision PD by 3mm and enter that as the single-vision reading PD value.

So it looks like DarkForest is correct, although he didn't include their "reduce the PD by 3" recommendation.
posted by RichardP at 12:11 PM on July 30, 2011 [2 favorites]

D'oh! Please, consider me sufficiently chastened about the fact that I've totally loaded that page before, and thus ignored it when it came up in searches for this question, thinking I'd already not found the answer there. And thanks, RichardP. ^_^
posted by SMPA at 2:04 PM on July 30, 2011

> At your high prescription value, beware polycarbonate lenses.... I like to stick to full thickness plastic

There are more than these two choices. In plastic, nothing beats CR-39 (abbe of 59, same as crown glass), but there are plastics that beat polycarbonate's abbe of 30.

High index plastic lenses are expensive, though. I got polycarbonate for my sunglasses because of the shatter resistance vs. errant frisbees.

"Lower Abbe numbers result in the presence of chromatic aberration (i.e., color fringes above/below or to the left/right of a high contrast object), especially in larger lens sizes and stronger prescriptions"
posted by morganw at 10:01 AM on August 6, 2011